Temperature (T) is a key parameter controlling the rheology of lava flows. Since hazardous behavior of eruptions prevents direct measurements of hot magmatic bodies [1], the temperature is mostly retrieved by measuring the infrared (IR) radiance of the lava flow [2, 3]. The determination of T is however subjected to important errors related to the poor knowledge of one of the most critical parameters, namely spectral emissivity (ε). In this study, we explored the temperature–emissivity relationship for basaltic magmas, mostly from the 2014–2015 Holuhraun eruption. We performed in situ spectral emissivity measurements at relevant magmatic temperatures (from room temperature up to 1800 K) over a wide spectral range (400–8000 cm−1) covering TIR, MIR and SWIR regions, using a non-contact IR emissivity apparatus [4]. To unravel the complex radiative behavior of basalts with temperature evolution, structural, chemical and textural analyses (SEM, EMPA, Raman spectroscopy, DSC, XRD, and TEM) were systematically performed. Our results show that spectral emissivity varies accordingly with temperature, wavenumber, and is greatly affected by micro-scale crystallization, emphasizing the effect of small change in silicate structure on magma radiative properties. Because of the multiphase nature of lava, each constitutive phase (glass, melt, crystal, vesicles) contribute differently to the spectral emissivity. The evaluation and quantification of the impact of these phases on effective thermal radiative properties is a key point to improve the accuracy of lava T determination. These new data will ultimately improve our knowledge of the complex lava flow properties that are crucial in thermo-rheological models for hazard assessment [5]. References: [1] Kolzenburg et al. 2017. Bull. Volc. 79:45. [2] Harris, A. 2013: Cambridge University press. 728. [3] Rogic et al. 2019 Remote Sens., 11, 662 [4] De Sousa Meneses et al. 2015. Infrared Physics & Technology 69. [5] Thompson and Ramsey, 2021, Bulletin of Volcanology, 83:41. Keywords: Spectral emissivity, temperature, IR spectroscopy, rheology, basalt